Development and Verification for the Control Method Using Surplus

Development and Verification for the Control Method Using Surplus Pressure of Primary Pumps in Chiller Plant Systems for Air Conditioning which Adopts Primary/Secondary Piping Systems Naomiki Matsushita1, Masahiro Fujimura1, Daisuke Sumiyoshi2, and Yasunori Akashi2 1 Aleph Networks Corporation, 2-3-12 Honjyo-Higashi, Kita-ku, Osaka 531-0074 JAPAN 2 Kyushu University, 6-10-1 Hakozaki Higashi-ku Fukuoka 812-8581 JAPAN Email: [email protected] Abstract: The primary/secondary piping systems are often employed in large chiller plant Systems. Normally, the primary flow becomes more than secondary flow, and the flow difference returns to a chiller via decoupler, which is common to primary flow loop (chiller side) and secondary flow loop (load side). It is a huge energy loss, because the primary pumps use their head to lead much flow to the decoupler. Therefore, we have developed new control method using surplus pressure of the primary pump to reduce the primary and secondary pumps’ energy. In this paper, we used this control method to the actual chiller plant buildings and verified its effectiveness. As a result, cold water conveyances, both primary loop and secondary loop, could be covered by only primary pumps during plant operating time, and the water conveyance power energy was reduced approximately 80%. Keywords: Primary/Secondary piping system, Water conveyance power, Control method for energy conservation Introduction: The primary/secondary piping systems shown in Figure-1 are often employed in large chiller plant Systems which have more than two chillers. Primary/secondary systems have two flow loops, the primary loop with chillers and the secondary loop with the loads, and each loop has primary pumps and secondary pumps for the cold water conveyance. The pipe, “Decoupler”, is common to these two flow loops and is connected between chillers and loads. The load, for example, air handling units in the secondary loop, has a two-way control valve, and so the secondary flow is changing by the load. On the other hand, the primary flow through the chillers is normally constant for maintaining chiller stability. The decoupler allows the pumps to operate at different flow rate. Normally, the primary flow becomes more than secondary flow, and the flow difference returns to a chiller via decoupler. It is a huge energy loss, if the big difference continues between primary flow and secondary flow, because the primary pumps use their heads to lead much flow to the decoupler. Therefore, we developed the control method using surplus pressure of the primary pump to reduce the primary and secondary pumps’ energy. This control method ensures the smallest flow to maintain the chiller operation and saves the water conveyance power by utilizing the surplus pressure of the primary pump to the secondary, though the flow has returned to a chiller via decoupler until now. The days which are required for a full capacity must be few in the buildings for business use, so the actual loads are in a low condition for the chiller capacity in most of a year. The load is a partial load for the operating chiller capability when the secondary flow becomes lower than primary flow, so the occurring frequency is very high. In this paper, we explain the “Control Method Using Surplus Pressure of Primary Pumps” and describe the substantiated effects of energy reduction in the actual plant systems. 1. Summary and Problems of Conventional Cool water conveyance Control Method The general control as a secondary AHU : Air handling unit Expansion tank conveyance control of cool water in AHU primary/secondary piping systems is a P ：Pressure Transmitter secondary pump inverter control to keep the ：Temperature Sensor secondary supply water header shown as ：Flowmeter ：Control Valve "SH-2" in Fig.1, and the operating pump P SH-1: Primary supply water header number control depending on the load flow SH-2 SH-2: Secondary supply water header rate. It is general that the set-point of INV RH-1: Return water header secondary supply header pressure for the SH-1 inverter control is set to the total head of its R-1,2: Chiller RH-1 rated flow by the secondary pump. For R-1 example, in case of the rated operating point 3 of secondary pump, 5m /min(flow rate) and R-2 300kPa (total head), shown in Fig.2, Fig.1 General Primary/Secondary Piping System 300kPa (about 30m-aqua) total heads are the target value for the inverter control. If the load flow is 1m3/min and is controlled to 300kPa target value, the frequency decreases to only approximately 48Hz because of the pump operating frequency characteristic figure shown in Fig.3. In this case, the 300kPa total head is not required, and the pump system applies excessive pressure to the secondary cool water loop. In fact, energy saving effects are not enough because the set-point becomes high condition in most time of a year by Fig. 2 Performance Curve of a Secondary Pump operating the designed value of maximum load flow, though the secondary pump is controlled by an inverter. 2. The Control Method Using Surplus Pressure of Primary Pumps in Chiller Plant Systems 2.1 Control Method Outline The control method that we developed enables practical use of primary pump Fig.3 Frequency Characteristic of Secondary Pump surplus pressure at the secondary cool water conveyance loop in order to reduce energy consumption of conveyance electric power. Instrumentation devices required for this control are only three devices shown in Fig.4, a pressure transmitter for measuring secondary supply header (Ps2) and primary supply header (Ps1), a pressure transmitter for P ：Pressure Transmitter ：Flowmeter measuring the end pressure at the end ：Temperature Sensor ：Control Valve Pｅ point of the chiller plant (Pe), and a P Terminal Expansion two-way control valve installed to the AHU Tank decoupler (V2, “decoupler valve” in Using secondary supply water header by the Estimated end pressure control this paper). The control module technique, when the end pressure is difficult to use component group to enable the control Ｔs Ps2 method using surplus pressure of P CHP2-n Required instrumentation Fl_1 primary pump is Table 1, and Fig.5 and V1 devices for this control Ｔr_2 Fig.6 are the module component group INV method P control flow charts. Fig.5 shows Ps1 V2 modules controlled by end pressure Ｔrh (Pe) and Fig.6 shows modules To_1 Ti_1 controlled by primary supply header R-1 CHP1-n pressure (Ps1). Under this control To_2 Ti_2 method, Fig.7 is a diagram showing R-2 correlation between the points of Fig. 4 Instrumentation Devices Required to the Control pipeline in a chiller plant system and Method Using Surplus Pressure of Primary Pumps the pressure value of each point in the pipeline. A chiller plant is located in the lower floor such as the basement in this diagram, and it shows the end load, air handling units installed in the most distant point, is located on the floors above the chiller plant in this case. The decoupler valve control adjusts the primary supply header pressure and increases a suction pressure on the secondary pump as much as possible (sown ih n (1), Fig.7). Then it adjusts the end pressure to the set-point by the inverter control of secondary pump (shown in (2), Fig.7). As a result, it enables to lower the inverter output and conveyance power energy consumption is reduced. Table 1: Control Point of Each Module No. Control Module Control Point 1 Inverter Control of Secondary Pump End Pressure (Pe) 2 Number Control of Secondary Pump End Pressure (Pe) 3 Pressure Relief Valve Control End Pressure (Pe) 4 Decoupler Valve Control Primary Supply Water Header Pressurure (Ps1) Fig. 5 Module Group Controlling by End Pressure (Pe) Decoupler Valve Control Primary Supply Water Header Pressure （Ps1） PID Control for the Decoupler Two-way Valve Control the pressure value of primary supply water header (Ps1) to be the set value of its pressure (Ps1_sp1) The set value of primary header pressure is set to keep the flow rate through a chiller more than 50% of the chiller rated flow rate. Decoupler Valve Output Fig. 6. Module Controlling by Primary Supply Header Pressure (Ps1) Fig. 7 Diagram about Each Pressure Value at Points of Pipeline in a Chiller Plant System 2.2 Each Control Module Details for the Control Method Using Surplus Pressure of Primary Pumps (1) Decoupler Valve Control Module This control module has two-way control valve and controls it for the primary supply header pressure (Ps1) to reach its target value (Ps1_sp1) by PID control. Generally, a role of primary pumps in the primary/secondary piping system is to keep a required flow rate for a chiller against the pressure loss of primary water flow loop. If the primary pump flow rate is more than the secondary one, in other words, the secondary loop flow is less than the rated flow rate of a chiller, the surplus flow returns to the chiller via a decoupler. (See Fig.8) In this situation, this control reduces the primary pump flow rate and raises its total head, if the decoupler valve is installed and appropriately adjusted. (See Fig.9) Accordingly, it enables to reduce the work of secondary pump owing to this surplus pressure. On the other hand, the chiller stops abnormally when the primary flow rate decreases too much because it is impossible to keep the minimum flow rate for the chiller operation. Therefore careful decision is required for the primary supply header pressure set-point under the decoupler valve control. It is required that the header pressure set-point of primary supply ensures a chiller minimum flow rate, even if all two-way valves at the secondary loads completely shut and the secondary side loop flow rate indicates “0”.

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